Migration And Transformation Of Arsenic,Selenium,and Lead During Combustion Of Sludge And Low Calorific Value Coal

With the rapid development of industrial economy,the output of urban sludge and low-calorific value coal is increasing.How to "reduce,stabilize,harmless and recycle" it has become an urgent problem to be solved.In this paper,the content of arsenic,selenium and lead elements in sludge,coal gangue,coal slime and weathered coal was accurately detected by the optimized microwave enhanced digestion method and hydride generation-atomic fluorescence spectrometry.The occurrence forms of arsenic,selenium and lead elements in sludge,Shengyang sludge,coal gangue and their combustion products were tested by chemical stepwise extraction method.The transformation characteristics of the occurrence forms of arsenic,selenium and lead in sludge and coal gangue before and after combustion were revealed.The release law of arsenic,selenium and lead elements during the separate combustion of sludge,uplifting sludge and coal gangue in a plant was further studied,and the effects of oxygen concentration and combustion temperature on the release characteristics were investigated.Combined with the demand and development trend of sludge blending technology in coal-fired units,the effects of the blending ratio of sludge and coal gangue on the release of arsenic,selenium and lead in the combustion process were investigated.The main work and conclusions of this paper are as follows:1.The contents of arsenic,selenium and lead in sludge,gangue,slime and weathered coal were accurately determined by microwave enhanced digestion and hydride generation-atomic fluorescence spectrometry.Compared with traditional wet digestion,microwave enhanced digestion is simpler,safer and less loss of elements.For the samples studied in this paper,the mixed acid solution of HNO3-HCl-HF with a volume ratio of 6:2:2 was selected as the digestion solution,in which the digestion time and temperature of the sludge were lower than that of low-calorific value coal.2.The distribution of the occurrence forms of arsenic,selenium and lead in sludge,Shengyang sludge and coal gangue and their combustion products was investigated by chemical stepwise extraction method,and the transformation characteristics of the occurrence forms of arsenic,selenium and lead in sludge and coal gangue before and after combustion were revealed.The occurrence forms of arsenic,selenium and lead in sludge of No.1 plant and uplifting sludge are similar,mainly in organic binding state(F5),and after full combustion in carbonate+sulfate +oxide binding state(F2),silicate+silicaluminate binding state(F3).The occurrence forms of arsenic,selenium and lead in coal gangue are mainly sulfide bound state(F4),carbonate+silicaluminate bound state(F3)and sulfide bound state(F4),accounting for 34.48%,35.61%and 36.46%,respectively.The occurrence forms of As,Se and Pb in the coal gangue after combustion are mainly silicate+silicaluminate bound state(F3),sulfide bound state(F4)and silicate +silicaluminate bound state(F3),and the contents are 18.6%,15.21%and 21.8%,respectively.3.The release law of arsenic,selenium and lead elements in the separate combustion process of sludge,uplifting sludge and coal gangue in a plant was studied.The release rates of arsenic,selenium and lead in sludge,uplifting sludge and coal gangue increased with the increase of combustion temperature and oxygen concentration.Under the same combustion conditions,the release rates of arsenic,selenium and lead in sludge were higher than that of coal gangue,and the release temperature range of arsenic,selenium and lead in sludge was lower than that of coal gangue.When oxygen concentration is 0%,the release of arsenic,selenium and lead elements is significantly inhibited.When oxygen concentration exceeds 10%,the release rate of arsenic,selenium and lead elements is gradually weakened by oxygen concentration.4.The release law of arsenic,selenium and lead elements in the mixed combustion process of sludge,uplifting sludge and coal gangue in a plant was studied.The release rate of As,Se and Pb in the mixture increases with the increase of sludge content,mainly because the coal gangue contains more fixed carbon,hydrated lime and Al2O3 than the sludge,which can inhibit the release of As,Se and Pb in itself and the samples composed with sludge.The increase in oxygen concentration and combustion temperature makes the reaction more violent and facilitates the decomposition of the compounds in the sample,which leads to the widening of the pores in the particles,making it easier for the compound molecules of arsenic,selenium and lead elements to be transported outwards and released into the gas phase.